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Back in the early days of HDTV development a number of analog based schemes were developed and proposed as a standard, but then someone figured out how to do digital video compression (MPEG) and then the FCC said that they wanted an all digital system for HDTV. I had actually worked on a 2 channel analog HD system, one channel the normal NTSC signal (for backwards compatability) and another channel with extra picture resolution, side panels and digital sound. It worked fairly well, but there would have been reception issues (differences of noise in one of the channels vs the other, likely one VHF and the extra info in UHF, alias problems from the progressive to interlace conversion to get the NTSC image for the first channel, and such).
Another backward compatable analog scheme was to try to make use of what was called the "Fukanuki hole" (Dr Fukanuki was a Japanese research enginner working for one of the big japanese TV makers, and determined that there was a set of spectral holes similar to that with luma and chroma (comb filter sort of thing). And that side panel information was to be modulated onto a "Fukanuki" subcarrier and merged into the composite video, but 12 dB down. The side panels were to be image info to make the 4:3 image into a 5:3 image, side panels stitched on both sides of the NTSC 4:3 image. Being 12 dB down was an attempt to be less visible in the 4:3 image, but the signal to noise would be that much worse for the side panel portion of the 5:3 image. Also, you would still see random crap swimming thru the 4:3 part of the picture. In NTSC, the chroma subcarrier is in the luma, but we get away with it because the colored parts of the picture and the chroma subcarrier patterns track each other when things move about in the image. This system didn't work and was forgotten quickly.
As for today's HDTV, there's 18 different resolution settings possible, but they all use MPEG-2 compression and decoding. And then there is another set of standards addressing the packetization of the digital bits and another on how to modulate an RF carrier to transmit it on a TV channel. Along the way, there is variable length encoding, where common bytes get shorter digital codes. If the TV channel picks a lower resolution setting, they can broadcast two independent programs at the same time. IIRC, use the lowest setting and they can do 6 programs.
Most HDTV receivers are not "multiscan" (like VGA monitors) but scan convert to whatever rate the display is built for. Some of the 18 flavors are done at 24 progressive frames a second (movies) but you can't display that at that rate on a CRT (too much flicker). The receiver up converts it to a faster display rate.
Just something more to get confused by when you're at the local "Crazy Eddie" electronics shop trying to select a good HD set....
(Crazy Eddie was a big chain around NYC, but has since folded. Something about stockholder fraud or such...).
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